The invention relates to a method for attenuating spurious signals in a transmitter and/or receiver in which radio signals are mixed to a second frequency, e.g. to an intermediate frequency (IF) or, in the case of so-called direct conversion, straight to the baseband, i.e. 0-Hz IF, and in addition the invention relates to a receiver. The invention thus pertains to a method defined in the preamble of claim 1 and a receiver defined in the preamble of claim 7.
When aiming at good linearity in radio reception, the biggest problem is usually the mixing of the received signal. Mixing is used for converting a high-frequency received signal down to a lower intermediate frequency. Attempts are made to attenuate in different ways the mixing results of even and odd orders which are brought about in the mixing in addition to the desired result. The mixing results of even orders are canceled by means of balanced or double-balanced mixer constructions. In the ideal situation, the even mixing results of two branches of a mixer cancel each other as being opposite. In practice, the attenuation caused by canceling is sufficient when the intermediate frequency is right and frequency-selective filtering is used. Then the frequencies causing the second order are attenuated already before the mixer down to a level where not too many of those frequencies will be generated in the mixer. However, there are receivers in which it is advantageous to use an intermediate frequency that might temporarily have a second-order spurious signal. Such a receiver is found e.g. in a multiple-frequency transceiver using one intermediate frequency and a common local oscillator.
Spurious responses will also be generated in a direct-conversion receiver, the worst being modulation-frequency interference at the receiver""s mixer output caused by a strong amplitude-modulated (AM) signal of another transceiver. This will appear even if the frequency of the interfering signal considerably deviated from the receiving frequency. These interferences are mainly caused by the second-order distortion component which contains a variable-level DC component proportional to the amplitude of the interference-causing signal. The variable-amplitude signal produces at the mixer output a signal which comprises a variable DC component and the frequency of which is identical with the varying of the amplitude. This signal is attenuated in accordance with the mixer""s balance accuracy and linearity. Sufficient attenuation can be achieved e.g. by using a mixer that has a good signal amplitude tolerance. In this way even spurious signals of a large amplitude will not cause a considerable interfering signal at the mixer output.
From the prior art it is known a so-called Gilbert cell which is widely used in integrated multiplier circuits of communications systems, especially in mobile communication devices. Multiplier circuits are used in integrated IF parts such as mixers and variable gain amplifiers.
A mixer can be based on a Gilbert cell. Such a mixer has to use large currents, high operating voltages and high local oscillator level to achieve a small relative portion of interfering signal and small effect on the mixer operating points. Additionally, the balance of the incoming RF signal, balance of the amplification of the different branches of the mixer, and the balance of the local oscillator are of great importance to the attenuation of spurious signals of even orders. The low voltage used nowadays because of the tendency to reduce power consumption causes that not all mixer types provide sufficient attenuation of spurious signals. Tolerances of the components used in the mixers are too wide to achieve a balance good enough at low voltages.
It is a problem with known radio receivers that single unbalanced mixers produce, in addition to the desired mixing result, spurious signals of both even and odd orders.
It is another problem with known radio receivers that spurious signals of even orders, which are generated in balanced mixers in addition to the desired mixing result, are insufficiently attenuated at low voltages.
It is an object of the invention to improve the attenuation of spurious signals of even orders so that an adequate end result will be achieved even with low operating voltages and smaller currents.
The method according to the invention is characterized by what is expressed in claim 1. The receiver according to the invention is characterized by what is expressed in claim 7. Preferred embodiments of the invention are described in the dependent claims.
The invention pertains to a method for attenuating spurious signals in a transmitter and/or receiver in which radio signals are mixed to a second frequency which may also be the baseband frequency. In accordance with the invention, mixing is balanced by adjusting transistor bias voltages and/or currents in the mixer circuit.
The invention also pertains to a receiver that comprises a mixer for mixing radio signals to a second frequency, which may also be the baseband frequency, and means for attenuating spurious signals while receiving radio signals. In accordance with the invention, it comprises balance-adjusting means of mixer circuit to adjust the balance of the circuit by adjusting transistor bias voltages and/or currents in the mixer circuit.
It is an advantage of the invention that in the mixing to a second frequency spurious signals of even orders caused by balance errors resulting from component tolerances considerably attenuated.